U.S. patent application number 13/868592 was filed with the patent office on 2013-11-21 for antenna configuration provides coverage.
This patent application is currently assigned to Telefonaktiebolaget L M Ericsson (publ). The applicant listed for this patent is Telefonaktiebolaget L M Ericsson (publ). Invention is credited to Martin Nils JOHANSSON, Stefan Johansson, Sven Petersson.
Application Number | 20130307752 13/868592 |
Document ID | / |
Family ID | 40345082 |
Filed Date | 2013-11-21 |
United States Patent
Application |
20130307752 |
Kind Code |
A1 |
JOHANSSON; Martin Nils ; et
al. |
November 21, 2013 |
Antenna Configuration Provides Coverage
Abstract
The invention provides an antenna arrangement for a wireless
communication system arranged to have at least one transmit mode
and at least one receive mode, the arrangement comprising at least
three directional antennas in an antenna configuration. Each
directional antenna is arranged to have an azimuthal radiation
pattern shaped as a beam, each beam covering an angular sector,
such that a combined radiation pattern of all beams in a first
transmit mode is arranged to provide a full 360.degree.
omnidirectional coverage. By combining localization and
polarization of the directional antennas an omnidirectional
radiation pattern substantially without null-depths in the
azimuthal plane can be created when the radiation pattern of the
directional antennas are combined.
Inventors: |
JOHANSSON; Martin Nils;
(Molndal, SE) ; Johansson; Stefan; (Romelanda,
SE) ; Petersson; Sven; (Savedalen, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget L M Ericsson (publ) |
Stockholm |
|
SE |
|
|
Assignee: |
Telefonaktiebolaget L M Ericsson
(publ)
Stockholm
SE
|
Family ID: |
40345082 |
Appl. No.: |
13/868592 |
Filed: |
April 23, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12997948 |
Dec 14, 2010 |
8432329 |
|
|
PCT/EP2008/057771 |
Jun 19, 2008 |
|
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13868592 |
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Current U.S.
Class: |
343/893 |
Current CPC
Class: |
H01Q 1/246 20130101;
H01Q 21/29 20130101; H01Q 21/24 20130101; H01Q 21/205 20130101 |
Class at
Publication: |
343/893 |
International
Class: |
H01Q 21/29 20060101
H01Q021/29 |
Claims
1. An antenna apparatus for a wireless communication system, the
apparatus comprising: a first directional antenna being arranged to
have a first directional beam covering a first angular sector; a
second directional antenna being arranged to have a directional
second beam covering a second angular sector that neighbours the
first angular sector, the first directional beam and the second
directional beam at least partially overlapping; and a third
directional antenna being arranged to have a third directional beam
covering a third angular sector, wherein the first directional beam
has a first polarization (p1), the second directional beam also has
the first polarization (p1), the third directional beam has a
second polarization p2, wherein p1 and p2 are at least
substantially orthogonal, the phase center of the first directional
antenna is located within a circle having a radius of less than
about 4 times lamda, wherein lamda is a mean wavelength in an
operating frequency band of the antenna apparatus, the phase center
of the second directional antenna is located within said circle,
the phase center of the third directional antenna is not located
within said circle, the distance between the phase center of the
first directional antenna and the phase center of the second
directional antenna is less than the diameter of said circle, and
the distance between the phase center of the first directional
antenna and the phase center of the third directional antenna is
greater than the diameter of said circle.
2. The antenna apparatus of claim 1, wherein said directional
antennas are connected to the same receiving line.
3. The antenna apparatus of claim 1, wherein the first and second
directional antennas are located in a substantially horizontal
plane.
4. The antenna apparatus of claim 1, wherein the directional
antennas are mounted on a common mast, tower, roof or roof-top or
mounted on walls or similar structures.
5. The antenna apparatus of claim 1, wherein the separation angle
between a pointing direction of the first directional antenna and a
pointing direction of the second directional antenna is
substantially 120 degrees.
6. The antenna apparatus of claim 5, wherein the separation angle
between a pointing direction of the first directional antenna and a
pointing direction of the third directional antenna is
substantially 120 degrees, and the separation angle between a
pointing direction of the second directional antenna and a pointing
direction of the third directional antenna is substantially 120
degrees.
7. The antenna apparatus of claim 1, wherein, in a second receive
mode, a separate receiving line is arranged to be connected to each
of the directional antennas, thus creating a sectorized coverage
for uplink.
8. The antenna apparatus of claim 1, further comprising: a fourth
directional antenna being arranged to have a fourth directional
beam covering a fourth angular sector; and a fifth directional
antenna being arranged to have a fifth directional beam covering a
fifth angular sector.
9. The antenna apparatus of claim 1, wherein the phase center of
the fourth directional antenna is not located within said circle,
and the phase center of the fifth directional antenna is not
located within said circle.
10. The antenna apparatus of claim 9, wherein the phase center of
the first directional antenna is located substantially a distance r
from an origin point, the phase center of the second directional
antenna is substantially located the distance r from the origin
point. the phase center of the third directional antenna is located
substantially a distance R from the origin point, the phase center
of the fourth directional antenna is located substantially the
distance R from the origin point, the phase center of the fifth
directional antenna is located substantially the distance R from
the origin point, and r<R.
11. A base station for communication with mobile terminals in a
telecommunications network equipped with an antenna apparatus
according to claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of patent application
Ser. No. 12/997,948, filed on Dec. 14, 2010, which is a 35 U.S.C.
371 national stage application of international patent application
no. PCT/EP2008/057771, filed on Jun. 19, 2008. The above mentioned
applications are incorporated by reference herein in their
entirety.
TECHNICAL FIELD
[0002] This invention relates to the technical field of
telecommunication networks and specifically to the field of
antennas for base stations in a cellular communications system.
BACKGROUND
[0003] There are a number of scenarios in mobile communications
where the desired cell structure and the desired number of cells
are time-dependent. For instance, some parts of a mobile
communications system may experience a high load during daytime and
a lower load at night. This means that the resource requirement can
be drastically different over the course of 24 hours.
[0004] Similarly, the long term average load in a mobile
communications system will typically increase over time, which
means that the overall load in a particular area will change. The
system will then have to be reconfigured to incorporate additional
resources, for example as realized when increasing the number of
cells.
[0005] Examples of antenna- and propagation-related solutions to
increase load capacity are higher-order sectorization and addition
of new sites, both solutions providing an effective cell split.
[0006] The solutions above are non-reversible in the sense that
once they are deployed, the system complexity and resource
allocation is permanently increased. There are no non-trivial ways
to reverse cell split using conventional base station
configurations.
[0007] U.S. Pat. No. 6,091,970 discloses a base station comprising
an arrangement of several directional antennas whose individual
azimuthal beam patterns achieve a substantially omnidirectional
coverage. In one illustrated embodiment the signal transmitted from
one base station transceiver is split in three signals which are
fed to an antenna configuration of three directional antennas so as
to provide an almost omnidirectional or "pseudo-omnidirectional"
pattern. All antennas in the antenna configuration use the same
polarization for transmit and receive and an additional diversity
receiver is using a different polarization. The main drawback with
this solution is that a number of sharp null-depths are created in
the "pseudo-omnidirectional" pattern which will cause areas of poor
or no coverage. The U.S. Pat. No. 6,091,970 includes phase shifters
whereby two of the transmitted signals can be shifted in phase.
However this solution only moves the interferometer pattern
resulting from the combined radiation pattern from the three
antennas. This means that the null-depths are moved but not
eliminated. There is a need to avoid the problem with
interferometer pattern causing null-depths that occurs when antenna
patterns with the same polarization are combined.
[0008] The effect of the phase shifters in U.S. Pat. No. 6,091,970
only works over a limited bandwidth which means that the solution
also has the disadvantage of being narrowband. As the phase
shifters are inserted in the output lines the phase shift effect
only works for the transmitted signals, i.e. it is a downlink
solution only.
[0009] U.S. Pat. No. 6,577,879 B1 describes how an antenna pattern
control is maintained by employing orthogonal polarization
orientation for every other beam. An advantage with the present
invention over U.S. Pat. No. 6,577,879 B1 is that it provides a
solution also to the problem of providing a combined,
omnidirectional radiation pattern without null-depths when
employing a solution with an odd number of beams from directional
antennas where each beam is covering an angular sector of a full
360.degree. omnidirectional coverage.
[0010] There is thus a need for an improved, reliable and low
complexity solution that eliminates the drawbacks of the existing
solutions.
SUMMARY
[0011] The object of the invention is to remove at least some of
the above mentioned deficiencies with prior art solutions and to
provide: an antenna arrangement; a method for an antenna
arrangement; and a base station equipped with the antenna
arrangement to solve the problem of providing an omnidirectional
radiation pattern substantially without null-depths when the
radiation pattern of any number of partially overlapping beams are
combined.
[0012] This object is achieved by providing an antenna arrangement
for a wireless communication system arranged to have at least one
transmit mode and at least one receive mode, the arrangement
comprising at least three directional antennas in an antenna
configuration. Each directional antenna is arranged to have an
azimuthal radiation pattern shaped as a beam, each beam covering an
angular sector, such that a combined radiation pattern of all beams
in a first transmit mode is arranged to provide a full 360.degree.
omnidirectional coverage. Said directional antennas are spatially
arranged such that the beams covering neighbouring angular sectors
partially overlap and such that the radiation patterns of all beams
are arranged to be combined by connecting the directional antennas
to the same transmitting line wherein: (a) at least two directional
antennas covering neighbouring angular sectors and with their phase
centres within a circle with a radius below two .lamda. are
arranged in a first cluster in which all directional antennas have
substantially the same polarization, where .lamda. is a mean
wavelength in the receive/transmit frequency band, (b) the antenna
arrangement comprises at least one cluster, (c) the polarization of
the separate directional antenna or the antenna cluster is
substantially orthogonal to the polarization of the separate
directional antenna or antenna cluster covering a neighbouring
angular sector, (d) the sum of antenna clusters and, separate
directional antennas not included in a cluster, is an even number,
(e) a directional antenna is part of one cluster only, in the same
antenna configuration, thus creating an omnidirectional azimuthal
radiation pattern substantially without null-depths.
[0013] The object is further achieved by providing a method for an
antenna arrangement in a wireless communication system having at
least one transmit mode and at least one receive mode, the
arrangement comprising at least three directional antennas in an
antenna configuration. Each directional antenna having an azimuthal
radiation pattern shaped as a beam, each beam covering an angular
sector, such that a combined radiation pattern of all beams in a
first transmit mode provides a full 360.degree. omnidirectional
coverage. Said directional antennas being spatially arranged such
that the beams covering neighbouring angular sectors partially
overlap and such that the radiation patterns of all beams are
combined by connecting the directional antennas to the same
transmitting line wherein: (a) at least two directional antennas
covering neighbouring angular sectors and with their phase centres
within a circle with a radius below two .lamda. are localized in a
first cluster in which all directional antennas have substantially
the same polarization, where .lamda. is a mean wavelength in the
receive/transmit frequency band, (b) the antenna arrangement
comprises at least one cluster, (c) the polarization of the
separate directional antenna or the antenna cluster is chosen to be
substantially orthogonal to the polarization of the separate
directional antenna or antenna cluster covering a neighbouring
angular sector, (d) the sum of antenna clusters and, separate
directional antennas not included in a cluster, is configured to be
an even number, (e) a directional antenna is checked to be part of
one cluster only, in the same antenna configuration, thus creating
an omnidirectional azimuthal radiation pattern substantially
without null-depths.
[0014] The invention also provides a base station for communication
with mobile terminals in a telecommunications network equipped with
an antenna arrangement according to any one of the antenna
arrangement claims.
[0015] The invention has the advantage to allow the antenna
configuration of a site to be adapted to different scenarios
without having to change the antenna installation.
[0016] Further advantages are achieved by implementing one or
several of the dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIGS. 1a-1e schematically show examples of sector antennas
and Tower Mounted Amplifier arrangements mounted on a mast.
[0018] FIGS. 2a-2b schematically show models of site scenarios.
[0019] FIG. 3 schematically shows azimuthal antenna radiation
patterns for a three-sector site.
[0020] FIGS. 4a-4h schematically show radiation patterns when
combining three antennas at different separation.
[0021] FIGS. 5a-5c schematically show three examples of radiation
patterns for a three-sector configuration.
[0022] FIGS. 6a-6b schematically show a model of an antenna
arrangement according to the invention in an embodiment for a
three-sector site.
[0023] FIGS. 7a-7d schematically show radiation patterns for an
antenna arrangement according to the invention with variations in
distance between antennas having different polarization.
[0024] FIGS. 8a-8h schematically show radiation patterns for an
antenna arrangement according to the invention with variation in
distance between antennas having the same polarization.
[0025] FIG. 9 schematically shows a configuration of five
directional antennas according to the invention.
[0026] FIG. 10 schematically shows a general antenna configuration
according to the invention.
[0027] FIG. 11 shows a block diagram of the inventive method.
[0028] FIG. 12 schematically shows a switching arrangement.
DETAILED DESCRIPTION
[0029] The invention will now be described in detail with reference
to the drawings.
[0030] The invention is concerned with an antenna arrangement, and
corresponding method, for a telecommunications network as e.g. a
cellular communication system. The antenna arrangement comprises a
number of directional antennas mounted for example to a tower or
mast and connected to a base station. The base station is
communicating with mobile terminals within the coverage of the
antenna arrangement. Each of the directional antennas has a
radiation pattern with a main beam covering an angular sector which
has a corresponding angular interval, in the azimuthal direction
around a vertical axis, being a portion of the total angular
coverage interval of the base station, with a certain overlap
between neighbouring angular sectors (or overlap between
neighbouring beams). An example of a common configuration is three
directional antennas with corresponding beams, each beam covering
approximately an angular sector of 120.degree., the configuration
providing a full 360.degree. coverage around the base station site.
The invention also includes a base station equipped with the
inventive antenna solution.
[0031] Each directional antenna covers one angular sector. The
directional antennas used can be sector antennas, as they are
optimized to cover a certain angular sector typically around
120.degree.. Each sector antenna, comprising at least one antenna
element, produces one beam for this certain angular sector. The
directional antenna can also consist of a number of antenna
elements being a part of e.g. an array antenna or other antenna
structure, and producing one beam covering one angular sector.
Although the invention can be implemented in applications with any
number of sectors, the problem addressed mainly exists for
applications in which the number of beams from the directional
antennas is an odd number equal to or greater than three. Also
other types of antennas can be used as long as they are producing
one beam for each sector. A common feature for all antenna types
used, is that the beams of neighbouring angular sectors are
partially overlapping.
[0032] Omnidirectional coverage of an antenna arrangement is
defined as an antenna arrangement having a radiation pattern
covering 360.degree. without null-depths, i.e. there are no angles
at which there will be poor or no coverage. The omnidirectional
antenna radiation pattern does not have to be isotropic, i.e. the
power received or the power transmitted does not have to be equal
in all directions.
[0033] FIG. 1 illustrates the principles of mounting the
directional antennas, in this example sector antennas, on a tower
or a mast. Furthermore, the invention is illustrated for a site
with three sector antennas having pointing directions separated by
120.degree., where either three-sector coverage or omnidirectional
coverage is desired. Henceforth a three-sector coverage, or
sectorized coverage, means that each directional antenna is
connected to a separate transmitting and/or receiving line and
omnidirectional coverage normally means that all directional
antennas carry replicas of the same signal on downlink, which can
be realized for example by having all directional antennas
connected to the same transmitting line. The power level of the
signals to each directional antenna can however differ e.g. by
inserting amplifiers as will be explained below. Downlink means
that the directional antennas work in transmit mode and uplink
means that the directional antennas work in receive mode. On uplink
an omnidirectional coverage can be achieved by connecting all
directional antennas to the same receiving line but the signal to
each directional antenna can differ depending on from which
direction the signal is received. This will be explained more in
detail in association with FIG. 2. Other installation scenarios and
site arrangements, for example with different number of antennas,
sectors and pointing directions, and with omnidirectional coverage
on downlink only and three sector coverage, i.e. sectorized
coverage, on uplink, are possible within the scope of the
invention. Different types of antennas can also be used as
described above. Two examples of arrangements are shown in FIGS. 1a
and 1b.
[0034] The invention can thus be used in both downlink and uplink
operation. In the description the invention is mainly exemplified
in downlink operation. Each example is however operational in both
uplink and downlink as described above.
[0035] FIG. 1a shows a single down-tilted sector antenna 101
mounted on an antenna tower 102. The sector antenna is connected
through a first transmission line 103 to a Tower Mounted Amplifier
TMA, 104, which in turn is connected to transmit/receive circuits
of a base station via a second transmission line 105. The sector
antenna 101 in this example covers an angular sector width of
substantially 120.degree. and the tower is equipped with three
identical sector antennas (only one antenna shown in FIG. 1a for
clarity reasons) with their pointing directions 116 separated with
120.degree., see FIGS. 1c-e.
[0036] FIG. 1b shows another example with a single modular high
gain sector antenna comprising two antenna elements 106 and 107
each connected to a combiner 108 through combiner transmission
lines 109. The combiner is connected to the TMA 104 through a third
transmission line 110 and then further to the base station
circuitry through the second transmission line 105. The antenna
elements 106 and 107 in this example covers an angular sector width
of substantially 120.degree. and the tower is equipped with three
identical pair of antenna elements (only one pair of antennas shown
in FIG. 1b for clarity reasons) with their pointing directions 116
separated with 120.degree., see FIGS. 1c-e.
[0037] The directional antennas mounted on a common tower, mast,
roof or roof-top or mounted on walls or similar structures do not
necessarily have to be identical but can have different performance
in e.g. terms of gain and beam-shape.
[0038] FIGS. 1c, 1d and 1e show top views of different arrangements
of the sector antennas 112 when mounted on a tower or mast with a
triangular 113, square 114 or circular 115, cross section. Pointing
direction 116 of each sector antenna is perpendicular to an antenna
aperture 117. The pointing directions are separated by a separation
angle 118. In the examples in FIGS. 1c-e the separation angle is
120.degree. between pointing directions of neighbouring
antennas.
[0039] For a number of reasons, for example zoning requirements and
cost (both Capital and Operational expenditures), it can be
advantageous to allow the antenna configuration of a site to be
adapted to different scenarios, without having to change the
antenna installation. During night when traffic flow normally is
low it can be advantageous to temporarily inactivate part of the
base station in order to save operational expenditures. When a new
base station is installed in can be advantageous to start up with a
minimum configuration of the base station, e.g. just one radio
chain, to save capital expenditures and then add on more radio
chains as traffic is increasing. A radio chain includes the
directional antenna and corresponding transmitting and receiving
line as well as electronics used specifically for the directional
antenna as e.g. a transceiver.
[0040] Two different models of site scenarios that use identical
antenna arrangements are shown in FIG. 2 for uplink and downlink
operation. FIG. 2a shows a conventional three-sector scenario
providing sectorized coverage with three transmitting/receiving
lines, each transmitting/receiving line connected to one
directional antenna each. The transmitting/receiving lines can e.g.
be part of three separate radio chains (one radio chain per
sector), each chain having a separate transceiver. FIG. 2b shows an
omnidirectional coverage scenario comprising only one
transmitting/receiving line being part of a single radio chain. The
single transmitting/receiving line is split into three
transmitting/receiving lines and each split transmitting/receiving
line is connected to one directional antenna each.
[0041] FIG. 2a is a top view of a first, second and third
directional antenna 201, 202 and 203 located in an X/Y-plane,
normally a horizontal plane, and configured in a three-sector
embodiment. The first 201 and second 202 directional antenna are
located a radius r, 204, from an origin 205. The third directional
antenna 203 is located a radius R, 207, from the origin 205. All
directional antennas have an antenna aperture 117 perpendicular to
the corresponding radius vector. The separation angle 118 between
neighbouring directional antennas is 120.degree.. The antenna
separation D1 between phase centres of the first and the second
directional antenna is indicated with arrow 206 and the antenna
separation D2 between phase centres of the first and the third
directional antenna is indicated with arrow 216. The phase centre
of a directional antenna, or any antenna, is defined as "the
location of a point associated with an antenna such that, if it is
taken as the centre of a sphere whose radius extends into the
far-field, the phase of a given field component over the surface of
the radiation sphere is essentially constant, at least over that
portion of the surface where the radiation is significant". The
first directional antenna 201 is connected to a first
transmitting/receiving line 208 from e.g. a first radio chain
through a first transmission line 211, the second directional
antenna 202 is connected to a second transmitting/receiving line
209 from e.g. a second radio chain through a second transmission
line 212 and the third directional antenna 203 is connected to a
third transmitting/receiving line 210 from e.g. a third radio chain
through a third transmission line 213. Each radio chain has its own
transceiver and has a certain capacity and available power
resource. When the capacity requirement is reduced, e.g. during the
night, it can be advantageous to temporarily inactivate part of the
base station without changing the antenna configuration in order to
save operational costs, for example as incurred due to power
consumption in transceiver and refrigeration equipment.
[0042] FIG. 2b illustrates the situation when all three directional
antennas are connected to the same transmitting/receiving line in
an omnidirectional coverage configuration. In this embodiment the
active electronics, i.e. primarily transceivers, in two radio
chains can thus be temporarily inactivated. The operating
transceiver is connected to a splitter/combiner 214. A fourth
transmitting/receiving line 215, comprising e.g. the
transmitting/receiving line 208, 209 or 210 coming from e.g. a
radio chain in a base station, is split in the splitter/combiner
into three split transmitting/receiving lines each connected to one
directional antenna through a the first transmission line 211 to
the first directional antenna 201, the second transmission line 212
to the second directional antenna 202 and the third transmission
line 213 to the third directional antenna 203. The phase of the
signals in one or more of the transmission lines, can be adjusted
with phase adjusters, such as true time delay units. The phase
adjusters can be used to fine tune the radiation pattern combined
from the radiation patterns of the individual directional antennas.
The phase adjusters are however optional and not required for the
invention. The signals in the split transmitting/receiving lines
can also optionally be amplified to compensate for the loss due to
the spitting of the signal of the fourth transmitting/receiving
line. The amplifiers can be located either in the transmission
lines or the transmitting/receiving lines. This compensation can be
implemented using for example TMAs on uplink or power amplifiers on
downlink, or both, using duplex arrangements, either connected to
the fourth transmitting/receiving line 215 or connected to the
transmission lines 211-213 or both.
[0043] The transmitting/receiving lines 208, 209, 210 and 215 in
FIGS. 2a and 2b can either be a combined transmitting and receiving
line or a separate transmitting and/or a separate receiving line,
i.e. it will be a transmitting line in transmit mode and a
receiving line in receive mode.
[0044] FIG. 2b thus shows an antenna configuration in a first
transmit mode and/or a first receive mode providing omnidirectional
coverage. FIG. 2a shows an antenna configuration in a second
transmit mode and/or a second receive mode providing sectorized
coverage.
[0045] Azimuthal, normally horizontal, radiation patterns of a
three-sector site, configured as shown in the scenario in FIG. 2a,
are plotted in FIG. 3. The radiation patterns 301-303 from
directional antennas 201-203 provide coverage, i.e., antenna gain,
in all directions, with dips in coverage along the sector borders
304-306 as illustrated in FIG. 3. This is called a sectorized
coverage with three effective angular sectors or three effective
radiation patterns or beams.
[0046] By reconfiguring the three-sector site to the scenario in
FIG. 2b, an omnidirectional azimuthal radiation pattern (providing
omnidirectional coverage) is generated. This omnidirectional
pattern is the result of a combination of the three separate
directional antenna patterns in FIG. 3. With the assumption that
the patterns have the same polarization and that all antennas carry
the same signal (on transmit), they must be added taking into
account both the amplitude and effective phase of the patterns,
that is coherently combined, with the effective phase being also a
function of antenna location.
[0047] The effects of antenna location are clearly illustrated in
FIG. 4, which shows the azimuthal radiation pattern, normally the
horizontal radiation pattern, resulting from feeding the three
directional antennas with the same (coherent) signal according to
the configuration in FIG. 2b with antenna separations D1=D2 and
radius r=R. All radiation patterns 4a-4h show patterns generated
when combining in phase three directional antenna radiation
patterns with the same polarization in all directions to generate
omnidirectional coverage. For antennas placed (unrealistically)
close together, the distances D1 and D2 between phase centres being
zero, the effective radiation pattern has a smooth omnidirectional
shape which provides coverage similar to that of the envelope
pattern of the three directional antennas in FIG. 3 according to
the configuration in FIG. 2a. This is shown in FIG. 4a. As the
antennas are moved apart in the azimuthal plane, the resulting
radiation pattern starts getting ripples, which develop into
angular intervals with severe gain drops, so called null-depths,
when the phase centres of the antennas are more than 1-2
wavelengths away from the common origin. In FIG. 4b the radii r and
R are 1/4 of a wavelength, in FIG. 4c 1/2 of a wavelength, in FIG.
4d 1 wavelength, in FIG. 4e 2 wavelengths, in FIG. 4f 5
wavelengths, in FIG. 4g 10 wavelengths and in FIG. 4h 20
wavelengths. For one example of a typical cellular communication
system, the frequency is around 1 GHz which corresponds to a
wavelength of 30 cm. For practical reasons, such as the
cross-sectional dimensions of the structure on which the antennas
are mounted, it is therefore often needed to use distances D1 and
D2 above 1-2 wavelengths. This becomes even more necessary for
higher frequencies used e.g. in the UMTS (Universal Mobile
Telecommunication System) band where the wavelength is around 15
cm.
[0048] Angular spread describes the property that signals
transmitted from one end of a wireless communications link appear
to emanate, on average, from an angular range or interval (the
width of which depends on the propagation environment, and distance
and direction between the two ends of the communications link, and
can be arbitrarily narrow) of directions when observed at the other
end of the communications link. From a radiation point-of-view,
angular spread can be thought of as a filter that should be
convolved with the antenna radiation pattern to get the effective
pattern for the given propagation environment. Therefore, radiation
pattern gain drop corresponds to loss of coverage when the
azimuthal or horizontal angular spread is narrower than the width
(at some acceptable relative gain level) of the angular interval
experiencing the gain drop, since the averaging effect due to
angular spread is insufficient to counteract the gain loss. The
larger the separation distance, the narrower the null-depth becomes
(the faster the ripple), and the pattern becomes
interferometer-like. Thus, for antennas spaced sufficiently far
apart as related to the angular spread of the given propagation
environment and antenna installation, effective omnidirectional
coverage may exist because of the averaging provided from angular
spread.
[0049] The conclusion is that the relative positions or location of
the antennas is a critical design factor if an antenna site is to
provide omnidirectional patterns using the sum of sector patterns
with the same polarization for directional antennas. But many
installations do not provide any (or much) choice with respect to
antenna position or location, which means that the combined pattern
is very much dependent on how the antennas are placed in relation
to each other at the specific installation site. This is true in
particular since the effective phase values of the radiation
patterns also depend on all the components in the radio chain, for
example amplifiers, filters, and feeder transmission lines.
[0050] The present invention introduces an antenna arrangement that
allows e.g. a three-sector antenna installation to be used for
omnidirectional coverage. This is the most common configuration but
the invention can also be used for configurations with any other
numbers (odd or even) of sectors, the number of sectors being at
least three. This will be explained further below. A basic concept
of the invention is to combine radiation patterns with different
polarizations and to combine radiation patterns with the same (or
similar) polarization and coherent signals for antennas that are
spaced close together to avoid the problems with radiation pattern
ripple, which may result in large angular regions having poor or no
coverage.
[0051] FIG. 5 shows a three-sector antenna configuration as in FIG.
3 with r=R=5A, where .lamda. is the mean wavelength in the
operating frequency band of the antenna. FIG. 5 further illustrates
how a basic concept of the invention based on using different
polarizations is applied to the patterns of two out of three
directional antennas in a three-sector site configuration where the
directional antennas are displaced radially five wavelengths from a
common origin. FIG. 5a shows three radiation patterns 501-503, or
beams, for directional antennas, each directional antenna covering
an angular sector, with uniformly spaced pointing directions 116 in
the azimuthal plane and fed with independent signals, thus without
coherent combining. FIG. 5b shows the resulting power pattern
501/503 when two co-polarized directional antennas are fed with
replicas of the same signal, with the pattern exhibiting strong
ripple due to constructive and destructive interference between the
radiation emanating from the two combined directional antennas.
FIG. 5c shows the power pattern resulting from applying one aspect
of the present invention, with the two combined antenna patterns
501/503 being configured to use different, essentially orthogonal
polarizations. FIG. 5c thus illustrates that by combining
orthogonal polarization patterns for partially overlapping beams of
neighbouring angular sectors a radiation pattern without
null-depths can be achieved.
[0052] The concept of using combination of radiation patterns with
different polarizations can be applied repeatedly for a given site
configuration with any number of antennas greater or equal to two,
the effective number of radiation patterns being reduced by one for
each combination, until two different effective patterns remain. If
these two effective patterns have different essentially orthogonal
polarizations, which corresponds to a site configuration with an
even number of sectors, in directions where the patterns produce
similar coverage, the patterns can be combined to get an effective
omnidirectional pattern. Thus for an even sector site
configuration, an effective omnidirectional radiation pattern can
be achieved by neighbouring angular sectors having always
substantially orthogonal polarizations. However, for an odd-number
sector site configuration this is not possible, as there will
always be two neighbouring angular sectors having the same
polarization. The invention now adds location as a further
parameter, above orthogonal polarization as described above, to be
used in the configuration of an antenna site. By suitable location
in a cluster, comprising two or more directional antennas with
neighbouring beams, these directional cluster antennas can have
substantially the same polarization. There can be one or several
clusters. By combining the principles of orthogonal polarization
and location, any number of angular sectors can be combined to
obtain an omnidirectional coverage as long as the sum of antenna
clusters and separate directional antennas not included in a
cluster is an even number. This will be explained further in
association with description of the figures below.
[0053] FIG. 6a shows a schematic model (top view) of the antenna
arrangement in an X/Y-plane. The beam of a first directional
antenna 601 and a second directional antenna 602, with the same
polarization `p1`, are combined. The splitter/combiner 214,
according to FIG. 2, may have a uniform or non-uniform power
splitting. The splitter/combiner 214 shall provide phase coherent
combination, taking into account directional antenna 601 (201),
transmission line 211, directional antenna 602 (202), transmission
line 212, such that the combined pattern does not exhibit
null-depths or that the null-depths are minimized. Furthermore the
beam of a third directional antenna 603 (203) with a non-identical,
substantially orthogonal polarization `p2`, is also combined in the
splitter/combiner 214, however without requirements on phase
coherency. This means that the pattern for the third directional
antenna 603 can be added as power, that is non-coherently, since
orthogonal polarizations are independent of each other, meaning
that it introduces no ripples to the effective omnidirectional
radiation pattern. The first and second directional antenna, with
polarization `p1`, are placed a radius r1, 606, and r2, 605, from
an imagined coordinate system origin 607 whereas the third
directional antenna, with non-identical polarization `p2`, is
placed a radius R, 608, from the same origin. The radius r1 is the
distance between the origin 607 and the phase centre of the first
directional antenna 601 and the radius r2 is the distance between
the origin 607 and the phase centre of the second directional
antenna 602. The radius R is the distance between the origin 607
and the phase centre of the third directional antenna 603. The
radius r1 and r2 are in this case the same but this does not
necessarily have to be the case. Antennas within a common cluster
should be placed in substantially the same plane, parallel to the
X/Y-plane. The distance D1, 609, between phase centres of the first
and second directional antenna should be less than about 3-4
wavelengths of the mean frequency in the combined transmit/receive
band. This can be seen from FIG. 4. When r.ltoreq.1-2.lamda. the
null-depths are not fully developed. In the configuration of FIG.
4, when r=.lamda., D1 becomes
2*sin60.degree.*.lamda..apprxeq.1,7.lamda. and when r=2.lamda., D1
becomes 3,5.lamda..
[0054] The first and second directional antenna, in the
configuration of FIG. 6, are said to comprise a cluster. A cluster
can include more than two antennas as will be shown below.
Antennas, covering neighbouring angular sectors and having
substantially the same polarization, that are located such that
their phase centres can be inscribed within a circle with a radius
of approximately 1-2 wavelengths .lamda., where .lamda. is the mean
wavelength in the receive/transmit frequency band, are defined to
belong to the same cluster. This circle is henceforth called the
.lamda.-circle. The radius of the .lamda.-circle should be below
2.lamda.. When two or more antennas are located close together it
is possible that one antenna .lamda. can belong to two or more
clusters depending on where the centre of the .lamda.-circle is
located. In that case there will be multiple possible antenna
configurations depending on which of the clusters antenna .lamda.
is included into.
[0055] FIG. 6b shows the first, second and third directional
antenna mounted on a tower 604 with a square cross section. This is
one installation scenario for which the present invention is well
suited, since the antenna separation distances become too large to
allow conventional pattern combination, i.e. not taking into
account both antenna polarization and antenna location.
[0056] One benefit of the present invention is clearly illustrated
in FIG. 7 which shows the azimuthal, normally the horizontal,
radiation pattern resulting from feeding three directional
antennas, such as sector antennas, arranged according to FIG. 6
with the same, that is replicas of the same, (coherent) signal for
different values of the radius R and with the third directional
antenna having substantially orthogonal polarization to the
polarization of the first and the second directional antenna. The
combined radiation pattern is, as will be shown, independent of the
location (radius R) of the third directional antenna. This means
that one can place the third directional antenna at a position, or
location, that is several wavelengths from the positions of the
first and second directional antenna, for example on the "opposite"
side of a tower as shown in FIG. 6b. This means that the
directional antennas can be located such as not to be obscured by
the structure to which they are mounted, in this case a tower. In
all radiation patterns in FIG. 7 the radius r is equal to a half
wavelength. In FIG. 7a the radius R=2 wavelengths, in FIG. 7b R=5
wavelengths, in FIG. 7c R=10 wavelengths and in FIG. 7d R=20
wavelengths. As can be clearly seen any value of R will generate
substantially the same radiation pattern. The third directional
antenna 603 in FIG. 6 can thus be placed at any distance from the
first and second directional antenna. For practical reasons it is
often more beneficial to use the possibility to locate the third
directional antenna far from the antennas in the cluster. However
the third directional antenna, having a substantial orthogonal
polarisation to the polarization of the first and second
directional antenna, can be located at any distance from the first
and second directional antenna, i.e. it can also be located within
the .lamda.-circle.
[0057] The requirements on the installation of the first and the
second directional antenna (the antennas being close together) are
illustrated in FIG. 8 which shows the azimuthal radiation pattern,
normally the horizontal pattern, resulting from feeding the three
directional antennas arranged according to FIG. 6 with the same
(coherent) signal, for different values of the radius r with R=10
wavelengths and with the third directional antenna having
substantially orthogonal polarization to the polarization of the
first and the second directional antenna. In FIG. 8a the radius r
for the first and the second directional antenna is 0 wavelengths
which is only theoretically possible, in FIG. 8b r=1/4 wavelength,
in FIG. 8c r=1/2 wavelength, in FIG. 8d r=1 wavelength, in FIG. 8e
r=2 wavelengths, in FIG. 8f r=5 wavelengths, in FIG. 8g r=10
wavelengths and in FIG. 8h r=20 wavelengths. As expected, the
behaviour in the angular region between the pointing directions of
the first and the second directional antenna is similar to the
behaviour for the case when radiation patterns with the same
polarization for all directional antennas are combined in a
configuration with r=R as illustrated in FIG. 4.
[0058] As can be seen in FIG. 8, null-depths are still not fully
developed when R.ltoreq.1-2.lamda.. In the configuration of FIG. 6
this corresponds to D1, the distance between phase centres of the
first and second directional antenna, being between
2*sin60.degree.*.lamda..apprxeq.1,7.lamda. and
4*sin60.degree..lamda..apprxeq.3,5.lamda.. Thus, an implementation
using the present invention should suitably be applied in such a
way that the antennas that can be placed with their phase centres
within the .lamda.-circle (as defined above) are identified and set
to have the same polarization when respective radiation patterns
are combined.
[0059] This invention thus allows multiple antennas to be connected
to the same transmitting/receiving line while generating radiation
patterns without null-depths, i.e., radiation patterns with limited
gain drop due to amplitude ripple, by using a combination of
antenna installation rules and polarization requirements. In
summary, this means that an antenna arrangement for a wireless
communication system arranged to have at least one transmit mode
and at least one receive mode, the arrangement comprising at least
three directional antennas in an antenna configuration, each
directional antenna being arranged to have an azimuthal radiation
pattern shaped as a beam, each beam covering an angular sector,
such that a combined radiation pattern of all beams in a first
transmit mode or in a first transmit and a first receive mode is
arranged to provide a full 360.degree. omnidirectional coverage.
Said directional antennas are spatially arranged such that the
beams covering neighbouring angular sectors partially overlap and
such that the radiation patterns of all beams are arranged to be
combined by connecting the directional antennas to the same
transmitting line or the same transmitting and receiving line
wherein: (a) directional antennas placed within the .lamda.-circle
shall use substantially the same polarization as illustrated in
FIGS. 4 and 8 and explained in association with these figures. This
means that at least two directional antennas covering neighbouring
angular sectors and with their phase centres within a circle with a
radius below two .lamda. are arranged in a first cluster in which
all directional antennas have substantially the same polarization,
where .lamda. is a mean wavelength in the receive/transmit
frequency band; (b) the antenna arrangement comprises at least one
cluster; (c) neighbouring beams having substantially orthogonal
polarization as illustrated in FIGS. 5 and 7 are combined without
causing null-depths. This means that the polarization of the
separate directional antenna or the antenna cluster is
substantially orthogonal to the polarization of the separate
directional antenna or antenna cluster covering a neighbouring
angular sector; (d) the sum of antenna clusters and, separate
directional antennas not included in a cluster, is an even number;
(e) a directional antenna is part of one cluster only, in the same
antenna configuration.
[0060] In this way an omnidirectional azimuthal radiation pattern
substantially without null-depths is created.
[0061] A separate directional antenna is a directional antenna not
included in a cluster.
[0062] Thus, this invention allows the same antenna configuration
to be used both for sectorized and omnidirectional coverage, i.e.,
both site scenarios in FIG. 2 can be supported using a single
antenna (and feeder, if desired) installation. However, in general,
the invention can be used also for a combination of sectorized and
omnidirectional coverage. The number of effective angular sectors,
after combination of one or several neighbouring beams, can be any
number between one and the number of sectors (or the number of
beams as there is one beam per angular sector) in the site
configuration. One sector corresponds to having a pattern that is
the combination of the radiation patterns of all beams, i.e. one
effective pattern. The solution for the switching arrangements
between sectorized and omnidirectional coverage, which is a
resource allocation operation involving signal routing and
power-up/power-down of base station equipment, is known and is not
part of the present invention. The switching arrangement is
schematically illustrated in FIG. 12 with switching means 1201
switching between the first transmit mode, 1202, and the second
transmit mode, 1203. A corresponding switching arrangement is used
for switching between the first and the second receive mode.
[0063] An advantage of the invention is that it provides a
low-cost, low complexity solution to the problem of generating a
combined effective radiation pattern substantially without
null-depths producing omnidirectional coverage using multiple
directional antennas such as sector antennas or an array antenna
connected to a common transmitting/receiving line. Each directional
antenna produces one beam for a certain angular sector. The array
antenna also produces one beam for each angular sector.
[0064] The invention is described for a three sector application
using three directional antennas. The directional antennas used can
be three-sector antennas, as they are optimized to cover a certain
angular sector typically around 120.degree.. Such an antenna
produces one beam for this certain angular sector. The directional
antenna can also e.g. be an array antenna producing one beam per
angular sector. However the invention can also be implemented in
configurations with any other number of sectors, odd or even, as
long as the number of sectors is above or equal to three. An
example of an embodiment with five directional antennas 901-905 is
shown in FIG. 9. All directional antennas, in this example
comprising sector antennas, have a directional radiation pattern,
or a beam, in the azimuthal plane, normally being the horizontal
plane. The first sector antenna 901 and the second sector antenna
902 have a radius r from the phase centers of the antennas to a
common origin 906. The third sector antenna 903, the fourth sector
antenna 904 and the fifth sector antenna 905 have a radius R from
the phase centers of the sector antennas to the common origin 906.
The first and the second sector antenna have the same polarization
p1 and have a distance between phase centers being less than 4
wavelengths. The phase centers of the first and second sector
antenna can therefore be inscribed within the .lamda.-circle and
they belong to the same cluster. The third, fourth and fifth sector
antennas are all placed far, i.e. more than 4A, from the first and
second sector antenna. The third sector antenna 903 has a
polarization p2 being substantially orthogonal to p1, the fourth
sector antenna 904 has the polarization p1 and the fifth sector
antenna 905 the polarization p2. This means that neighbouring
sector antennas to the cluster antennas having the same
polarization p1, have a substantially orthogonal polarization p2.
When all five sector antennas are connected to the same
transmitting/receiving line and the antenna patterns from the five
sector antennas are combined there will be no interferometer
patterns in the sector borders between the first and third sector
antennas and between the second and fifth sector antenna as the
neighbouring third and fifth sector antennas have a substantially
orthogonal polarization to the cluster antennas. The forth sector
antenna 904 has substantially the same polarization p1 as the
cluster antennas. There will be no interferometer patterns in the
sector borders between the fourth and third and the fourth and
fifth sector antenna as the third and fifth sector antennas have a
polarization p2 being substantially orthogonal to the polarization
p1 of the fourth sector antenna. As the fourth sector antenna does
not have a sector border with the first and second sector antenna
there will also not be an overlapping radiation pattern between the
fourth and the first or the fourth and the second sector antenna as
the antenna patterns for all sector antennas are directional and
thus there will be no interferometer pattern when the radiation
patterns from the fourth, first and second sector antenna are
combined although they have the same polarization p1. The only
possible overlapping of radiation patterns from the fourth and the
first and the fourth and the second sector antenna is the backlobe
pattern of the fourth sector antenna which could overlap with the
radiation patterns of the first and second sector antennas. The
backlobe is however typically 25-40 dB below the level of the main
lobe for typical sector antennas and thus has a negligible
influence when the radiation patterns are combined. When there are
more than three sector antennas in the antenna arrangement, and an
omnidirectional pattern shall be produced through feeding the
directional antenna/s with the same signal, the cluster antennas
covering neighboring sectors shall have substantially the same
polarization, and the cluster shall have neighbouring antennas or
antenna clusters with a substantially orthogonal polarization. The
cluster can comprise more than two directional antennas as long as
their phase centers can be inscribed within the .lamda.-circle. The
antenna configuration can comprise one or several clusters. An
antenna can only be part of one cluster in the same antenna
configuration.
[0065] The antennas do not have to be displaced along their
respective main beam pointing direction, as represented by radial
displacement from a common origin in the direction of vectors
normal to the apertures of the antennas, as shown in FIGS. 1, 2, 6
and 9. FIG. 10 shows the directional antennas displaced in an
X/Y-plane in a more general configuration. The first directional
antenna 1001 and the second directional antenna 1002 belong to a
cluster and have substantially the same polarization. The third
directional antenna 1003 is placed far from the other two
directional antennas and has a different polarization which is
substantially orthogonal to the polarization of the first and the
second directional antenna. As shown in FIG. 7 the distance to the
third directional antenna having a different polarization than the
first and the second directional antenna is not critical and the
third directional antenna can actually be placed at any distance
from the other two directional antennas. The first directional
antenna can be placed at point X1/Y1 with an angle .phi.1 to the
Y-axis, the second directional antenna at point X2/Y2 with an angle
(D2 to the Y-axis and the third directional antenna at a point
X3/Y3 with an angle .phi.3 to the Y-axis. The directional
co-polarized antennas shall be placed in substantially the same
X/Y-plane which e.g. can be the horizontal plane. As mentioned
earlier the characteristics of each directional antenna can differ.
The directional antennas can differ in characteristics such as e.g.
antenna gain, azimuth and elevation beam width, and elevation
pointing direction.
[0066] The invention also includes a method for an antenna
arrangement comprising the following steps as illustrated in FIG.
11: (a) localizing, 1101, directional antennas in a first cluster.
At least two directional antennas covering neighbouring angular
sectors and with their phase centres within a circle with a radius
below two .lamda. are arranged in a first cluster in which all
directional antennas have the same polarization, where .lamda. is a
mean wavelength in the receive/transmit frequency band. The antenna
arrangement comprises at least one cluster; (b) choosing
substantially orthogonal polarization, 1102, for overlapping beams
of neighbouring angular sectors. The polarization of the separate
directional antenna or the antenna cluster is substantially
orthogonal to the polarization of the separate directional antenna
or antenna cluster covering a neighbouring angular sector; (c)
configuring, 1103, the antenna arrangement such that the sum of
antenna clusters and, separate directional antennas not included in
a cluster, is an even number; (d) checking, 1104, that one
directional antenna is part of one cluster only, in the same
antenna configuration.
[0067] The invention also provides a base station for communication
with mobile terminals in a telecommunications network equipped with
an antenna arrangement according to any one of the claims 1-11.
[0068] The embodiments used to illustrate the invention correspond,
on downlink, to each antenna radiating the same amount of power,
thus the antenna patterns can be combined taking into account only
the gain of the antennas. In general, the invention allows the
combination of the radiation patterns from antennas radiating
different amounts of power, with the antennas having identical or
different radiation patterns corresponding to controlled variations
of the azimuthal angular sector coverage.
[0069] The radiation patterns used to illustrate the effects of
combining multiple radiation patterns to combined effective
patterns are to be interpreted as free space radiation patterns,
i.e., radiation patterns that are only obtainable in an ideal radio
wave propagation environment such as free space or in high-quality
antenna measurement ranges. In general, the invention is applicable
to arbitrary radio wave propagation environment, which exhibit
varying degrees of angular spread.
[0070] The invention is not limited to the embodiments above, but
may vary freely within the scope of the appended claims.
* * * * *